THE SCYPHOMEDUSAE 



67 



Order 4. Discomedusae. 



Definition. — Scyphomedusae with four peri'cadial and four 

 interradial (sometimes more) tentaculocysts. The radial canals 

 are either broad pouches or fine canals, and are often very 



Later development of Chrysaora and Aurelia (after Clans). A, scyphistonia of Clirysaora, 

 with four perradial tentacles and horny basal perisarc. 7?, oral surface of later stage of 

 scyphistonia of Aurelia, with coniniencenient of four interradial tentacles. The quadrangular 

 mouth is seen in the centre ; tlie outline of the stomach wall, seen by transparency around it, 

 is nipped in four places iuterradially to form the four gastric ridges. (', oral surface of a sixteen- 

 tentacled scyphistonia of Aurelia. The four gastric interradial ridgi's are seen through the 

 mouth. D, first constriction of the Aurelia scyphistonia to form thi' jiile of eiiliyrae or young 

 medusae (see Fig. 7). The single epliyra carries the sixteen scyphistonia tentacles, which will 

 atrophy and disappear. The four longitudinal gastric ridges are seen by transparency. E, young 

 ephyrajust liberated, showing the eight bifurcate arms of the disc and the interradial single 

 gastral tilaments. F, ephyra developing into a medusa by the growth of the adradial regions. 

 The gastral tilaments have increased to three in each of the four sets. A, margin of the mouth ; 

 Ad, adradial radius ; /•', gastral tilament ; In, interradial radius ; ./(;, adradrial gastral canal ; 

 JR = K^, adradial lobe of the disc ; A', lappet of a perradial ann ; M, stomach wall ; Aht, muscle 

 of the mesentery ; Mir, mesentery ; Ms, mesodenn ; 0, tentaculocyst ; P, perradial radius ; 

 i£2, interradial radius ; iJ3, adradial radius ; SG, commencement of circular canal. 



numerous ; they are not separated from each other by laminar 

 mesenteries, and a well-marked endoderm lamella unites them. 

 Development showing alternation of an asexual scyphistomoid 

 with a sexual medusoid generation. 



The Discomedusae are probably all a metagenetic hydroid- 

 like form alternating with a sexual medusoid generation. In 



